Humans and other animals are a rich source of mechanical power. In general, this mechanical power is derived from chemical energy. The chemical energy required for a muscle or group of muscles to perform a given activity may be referred to as the “metabolic cost” of the activity. In humans and other animals, chemical energy is derived from food. Food is generally a plentiful resource and has a relatively high energy content. Humans and other animals exhibit a relatively high efficiency when converting food into chemical energy which then becomes available to muscles for subsequent conversion into mechanical energy. Mechanical power generated by humans and other animals can be efficient, portable and environmentally friendly.
As a consequence of the attractive characteristics of human power, there have been a variety of efforts to convert human mechanical power into electrical power, including:                U.S. Pat. No. 1,472,335 (Luzy);        U.S. Pat. No. 1,184,056 (Van Deventer);        U.S. Pat. No. 5,917,310 (Baylis);        U.S. Pat. No. 5,982,577 (Brown);        U.S. Pat. No. 6,133,642 (Hutchinson);        U.S. Pat. No. 6,291,900 (Tiemann et al.).        
A subset of the devices used to convert human mechanical power into electrical power focuses on energy harvesting—the capture of energy from the human body during everyday activities. Examples of disclosures relating to energy harvesting include:                Starner, T., Human-powered wearable computing. IBM Systems Journal, 1996. 35(3-4): 618-629;        Chapuis, A. and E. Jaquet, The History of the Self-Winding Watch. 1956, Geneva: Roto-Sadag S. A.;        Shenck, N. S. and J. A. Paradiso, Energy scavenging with shoe-mounted piezoelectrics. IEEE Micro, 2001. 21(3): 30-42;        Kymissis, J., et al. Parasitic Power Harvesting in Shoes. in Second IEEE International Conference on Wearable Computing. 1998: IEEE Computer Society Press;        Antaki, J. F., et al., A gait-powered autologous battery charging system for artificial organs. Asaio J, 1995. 41(3): M588-95;        Gonzalez, J. L., A. Rubio, and F. Moll. A prospect on the use of piezolelectric effect to supply power to wearable electronic devices. in ICMR. 2001. Akita, Japan;        Moll, F. and A. Rubio. An approach to the analysis of wearable body-powered systems. in MIXDES. 2000. Gdynia, Poland;        Drake, J., The greatest shoe on earth, in Wired. 2001. p. 90-100;        Niu, P., et al. Evaluation of Motions and Actuation Methods for Biomechanical Energy Harvesting. in 35th Annual IEEE Power Electronics Specialists Conference. 2004. Aachen, Germany: IEEE.        U.S. Pat. No. 6,768,246 (Pelrine et al.);        US patent publication No. US2004/0183306 (Rome);        U.S. Pat. No. 6,293,771 (Haney et al.);        
Energy may be harvested from the movement of body joints of humans and other animals by converting mechanical energy derived from such movement to electrical energy. Activities where body joints move cyclically, such as walking, jogging, and running, for example, present opportunities to continually harvest energy from moving body joints. In some energy harvesting apparatus and methods, a generator driven by joint motion is coupled to a constant electrical load. Since the instantaneous mechanical power of body joints during cyclical activities typically varies over the period of each cycle, both the electrical power supplied to the load and the forces applied to the body joint may be time-varying over each cycle. In some circumstances, the variations of delivered electrical power and/or forces applied to body joints that occur in this arrangement may be undesirable, for reasons such as efficiency and user comfort.
There is accordingly a desire for improved apparatus and methods for harvesting energy from cyclical motion of body joints of humans and other animals.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.